Synthesis and Characterization of Squaraine-Based Photocrosslinkable Resists for Bulk Heterojunction Solar Cells

Article abstract of DOI:10.1002/ejoc.201600552

Organic photovoltaic devices (OPVs) are rapidly approaching the commercialization threshold thanks to continuous advances in conversion efficiencies and device durability. Even though the design of high performing active dyes (polymeric and molecular) has

Full text of DOI:10.1002/ejoc.201600552

DOI: 10.1002/ejoc.201600552
Full Paper
Organic Photovoltaics
Synthesis and Characterization of Squaraine-Based
Photocrosslinkable Resists for Bulk Heterojunction Solar Cells
Riccardo Turrisi,[a] Luca Mascheroni,[a] Mauro Sassi,[a] Myles Rooney,[a] Nunzio Buccheri,[a]
Riccardo Ruffo,[a] Antonio Facchetti,[b,c] and Luca Beverina*[a]
Abstract: Organic photovoltaic devices (OPVs) are rapidly ap- This paper proposes the use of functional photoresists based
proaching the commercialization threshold thanks to continu-
ous advances in conversion efficiencies and device durability.
Even though the design of high performing active dyes (poly-
meric and molecular) has dominated the literature, the formula-
tion of strategies improving stability and processability is at
least as important as the achievement of record efficiencies.
on acrylic end-capped squaraines as the photoactive layer of
OPV devices. Our results show that the post-deposition photo-
polymerization process leads to insoluble photoactive layers
that retain sizeable power conversion efficiencies, improved
with respect to those of the parent unexposed materials.
Introduction
Recently, the use of photocrosslinkable donor or acceptor
derivatives for photovoltaics gained attention as a viable tool
for stabilizing morphology over time and thermal stress. In par-
ticular, the chemistry of azido compounds was successfully
demonstrated on polymer materials, and, although with lower
efficiencies, on molecular materials as well.[25,36–41] One draw-
back of azide photochemistry lies in the photogeneration of
nitrene species. Such species are extremely reactive and thus
scarcely selective, in principle leading to reactions both within
the donor domain and with the PCBM acceptor material.
Over the last two decades, organic solar cells based on binary
mixtures of electron-donor (p-type) and electron-acceptor (n-
type), either molecular or macromolecular semiconductors,
have been the focus of considerable academic and industrial
interest.[1–6] Besides the development of active compounds
having an acceptable performance/synthetic accessibility trade
off,[7] one of the key tools to achieving high efficiencies is the
ability to control, at the nanoscale, the phase separation of p-
and n-semiconductors into interpenetrated domains, the so-
called Bulk Heterojunction morphology (BHJ).[8–10] The litera-
ture reports several approaches, including solubilizing chain en-
gineering,[11–16] use of mixtures of solvents,[4] use of high boil-
ing point additives,[17–22] nucleating agents,[23] supramolecular
approaches[24] as well as extensive optimization of processing
conditions (donor:acceptor ratio, deposition technique, concen-
tration, use of different buffer layers, annealing).[25–33] As the
BHJ morphology is a kinetic state, extensive thermal stress of-
ten leads to performance degradation over time. Also, some of
the most commonly employed post-processing treatments –
like solvent annealing – are poorly compatible with roll-to-roll
processing.[34,35]
The aim of this paper is to demonstrate the proof of principle
behind cinnamic ester photodimerizations and acrylate photo-
crosslinking as viable options for generating functional small
molecule-based OPV devices. The first approach (photodimeri-
zation) offers the advantage of potential selectivity and requires
no photoinitiator. The second approach (photocrosslinking)
represents one of the standard tactics in photoresist chemistry
offering a high degree of control over the photopolymerization
process.
We decided to target small molecules over polymers in order
to be able to carry out a through purification prior to photo-
crosslinking studies. The choice of squaraines (the 1,3-dicon-
densation products of squaric acid and two equivalents of elec-
tron rich precursors) among all possible donor materials de-
scribed in the literature is due to the fact that such compounds
offer a good trade off between synthetic accessibility and over-
all conversion efficiencies.[32,42–55] In particular, we considered
one of the simplest squaraine structures – the anilino-squar-
aines – to demonstrate the method (thereby providing proof of
concept data) rather than focusing at this juncture on attaining
particularly high conversion efficiencies. Also, to the best of our
knowledge, this is the first attempted synthesis of photocross-
linkable squaraines. Scheme 1 shows the chemical structure of
[a] Department of Materials Science, University of Milano Bicocca,
Via Roberto Cozzi 55, Milano 20125, Italy
E-mail: luca.beverina@unimib.it
www.mater.unimib.it
[b] Department of Chemistry and the Materials Research Centre, Northwestern
University
North Sheridan Road, Evanston, Illinois 60208, USA
[c] Polyera Corporation,
8045 Lamon Avenue, Skokie, Illinois 60077, USA
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201600552.
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Scheme 1. Chemical structure of the methacrylic (A2X, A4X), acrylic (A2XOH, A4XOH) and cinnamic (C2X, C4X)-functionalized squaraine derivatives employed
in our photocrosslinking experiments.
the cinnamic and acrylic derivatives we designed and character- pounds 1 and 2 were condensed with methacryloyl chloride in
ized.
the presence of triethylamine to give functionalized derivatives
5 and 6; these products were eventually condensed with
squaric acid in iPrOH/(EtO)3CH to give A2X and A4X in 17 %
and 27 % yields, respectively.
Results and Discussion
To explore the applicability of the photocrosslinking protocol
to molecules bearing additional functional groups, we also pre-
Design and Synthesis
In principle, both cinnamic and acrylic derivatives could be suc- pared hydroxyl-functionalized derivatives A2XOH and A4XOH.
cessfully employed for photocrosslinking experiments. We de- Worth noting is that the presence of a hydroxyl group adjacent
signed and synthesized derivatives C2X and C4X bearing two to the squarylium core generally leads to higher conversion effi-
and four cinnamic residues respectively, as well as derivatives ciencies.[57] Derivatives A2XOH and A4XOH were prepared
A2X and A4X bearing the same number of acrylic residues.
Scheme 2 shows the synthetic approach to cinnamic deriva-
tives: 2-[ethyl(phenyl)amino]ethanol (1) and N-phenyldiethanol-
amine (2) were condensed with cinnamic acid according to a
Fisher acid-catalysed reaction to give anilines 3 and 4, respec-
tively. The reaction is very slow and such derivatives can be
more conveniently synthesized starting from cinnamoyl chlor-
ide under alkaline conditions.[56]
(Scheme 3) starting from 3-aminophenol 7. The latter was alkyl-
ated first with bromooctane, to give N-octyl-3-aminophenol 8,
and then with 6-bromohexyl acrylate, to give derivative 10.
Compound 10 was eventually converted to squaraine A2XOH
by reaction with squaric acid in the usual iPrOH/(EtO)3CH mix-
ture. Double alkylation of 7 with excess of 6-bromohexyl acryl-
ate yielded diacrylated aniline 10 which was then directly con-
verted to squaraine A4XOH by refluxing in the iPrOH/(EtO)3CH
mixture. All functionalized squaraines can be stored at room
temperature indefinitely.
Scheme 2. Synthetic protocol leading to: a) Cinnamic derivatives C2X and
C4X. b) Methacrylic derivatives A2X and A4X.
The latter were subsequently condensed with squaric acid in
2-propanol and in the presence of excess ethyl orthoformate to
give squaraines C2X and C4X in 27 % and 10 % yields, respec-
tively. Regarding the acrylic derivatives (Scheme 2, b), com-
Scheme 3. Synthetic protocol leading to acrylic derivatives A2XOH and
A4XOH.
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UV/Vis and Electrochemical Characterization
Photocrosslinking Tests
Table 1 and Figure S1 of the Supporting Information show that Prior to proceeding with the photopolymerization tests, we
the nature and number of functionalization moieties bear little characterized by means of a coupled Differential Scanning Cal-
influence on the UV/Vis absorption and emission properties of orimetry (DSC)/Thermogravimetric Analysis (TGA) the thermal
the squaraine compounds we prepared. All compounds feature stabilities of A4X and C4X. This was done in order to rule out
a typical cyanine-like strong and narrow absorption band possible thermal polymerization processes. Figure S4 of the
within the 630–650 nm window, with a mirror emission band Supporting Information shows that both compounds are stable
and very little Stokes Shift. In the thin film form, aggregation up to 250 °C; as such, no spontaneous thermal polymerization
effects sizeably broaden such characteristic narrow absorptions, can happen within the thermal range explored in the present
as shown in Figure 1 for derivative A4XOH. All other com- study.
pounds showed comparable behaviours.
We first characterized the photocrosslinking behaviour of
cinnamic derivatives, as this particular crosslinking approach
can be performed without using a photoinitiator. Unfortunately,
neither C2X nor C4X proved to be reactive under UV irradiation
at 254 or 365 nm (standard TLC Wood lamp). Because cinnamic
acid dimerization is competitive with the cinnamic double bond
isomerization (a process favoured at low irradiation intensities),
we exposed a C4X film to a stronger UV source (50 mW cm–2)
for up to 30 min. Even in this case, we did not observe any
change in the chemical structure (as asserted by NMR analysis
of the dissolved thin film) or in the solubility. Cinnamic acid
dimerization is a topochemical reaction requiring appropriate
orientation and a distance smaller than 4.2 Å between the two
involved π orbitals, a criterion likely not met by our mol-
ecules.[58]
Table 1. UV/Vis absorption and emission maxima (in CH2Cl2), molar extinction
coefficient and electrochemically derived HOMO and LUMO energies for
squaraine derivatives.
λmax
[nm]
λmax em.
[nm]
ε
HOMO
[eV][a]
LUMO
[eV][a]
[L mol–1 cm–1
]
A2X
A4X
A2XOH
A4XOH
C2X
629
622
638
649
630
625
644
638
669
664
645
640
240.000
250.000
240.000
230.000
270.000
290.000
–5.33
–5.24
–5.10
–5.13
–5.18
n.s.[b]
–3.34
–3.27
–3.16
–3.16
–3.21
–
C4X
[a] See experimental details. [b] n.s.: not soluble.
We thus focused our attention on acrylic derivatives, in this
case selecting an appropriate photoinitiator. After a few explor-
ative trials, we decided to use Irgacure 651 (2,2-dimethoxy-1,2-
diphenylethan-1-one) as the photoinitiator. The absorption
spectrum shows a peak at 340 nm, a region where the squar-
aine absorption is minimal (thus avoiding possible filtering ef-
fects). Figure 2 shows a test we carried out with A4XOH. A
blade-coated film obtained from a 3:97 ratio (wt:wt) Irgacure
651:A4XOH solution in 1,2-dichloroethane (10 mg/mL) was par-
tially protected with a black mask and exposed, under nitrogen
flux, to the 365 nm UV light of a Wood lamp for 45 min. The
mask was removed and the film, immersed, and then sonicated
in an acetone bath. Derivative A4XOH is freely soluble in acet-
Figure 1. UV/Vis absorption spectra of derivative A4XOH in solution (dots)
and as a spin-coated film before (dashed line) and after (continuous line) UV
irradiation.
We also carried out cyclic voltammetry characterizations on
derivatives A2X and A2XOH as representative compounds in
the series. As shown in Figure S3 of the Supporting Information,
acrylic derivatives feature two reversible one-electron oxidation
processes and one irreversible reduction. HOMO and LUMO lev-
els were calculated according to the equation shown in the
Experimentals section. The electrochemical HOMO and LUMO
energies were found to be consistent with those reported in the
literature for similar compounds.[57] These HOMO and LUMO
energies were also found to be well suited to provide appropri-
ate level alignments with respect to PC61BM (having HOMO an
LUMO levels placed at –6.1 and – 4.2 eV, respectively). Table 1
summarizes relevant UV/Vis and electrochemical data for all
compounds.
Figure 2. a) Blade-coated film of derivative A4XOH after exposure to UV light
through a star shaped mask; b) Development in acetone; c) Final rinsed sam-
ple.
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one, and, in fact, the unexposed portion of the film was com- laminated, yet 70 % of the original squaraine absorption was
pletely washed away.
preserved.
Conversely, the exposed area became totally insoluble in the
developing solvent, thus confirming that the polymerization/
crosslinking process took place. It should be noted that the
comparison (Figure 1) of the UV/Vis absorption spectra of
A4XOH before (solid black line) and after (dashed line) photo-
crosslinking does not show any signs of photoinduced degrada-
tion. This crucial observation rules out the possible involvement
of the squarylium core double bond in the radical polymeriza-
tion process.
We further characterized the exposed and unexposed films
by means of FTIR spectroscopy working in an ATR configuration.
Figure 3 shows that the band at 1677 cm–1, previously attrib-
uted to the stretching of the acrylic double bond, sizeably
weakens after a 30 min exposure.[59]
AFM Morphological Characterization
Due to the results of photocrosslinking tests, we carried out
further investigations on A2X, A2XOH and A4XOH acrylic de-
rivatives alone. Derivative A4X was not further investigated due
to its very poor solubility and film-forming properties. We
started by characterizing the behaviour of pure photocrosslink-
able squaraine thin films on top of Molybdenum oxide (MoOx),
an emerging electron-blocking layer.[19,48] Thus, we spin-coated
chloroform solutions of squaraines (10 mg/mL, spinning at
900 rpm), containing 3 % by weight Irgacure 651. These thin
films were irradiated under a nitrogen flux at 365 nm for 30 min
and characterized by AFM before and after development. Fig-
ure 5 shows the AFM scans of A2X films taken at different prep-
Figure 3. FTIR-ATR absorbance of a thin film of pristine and photocrosslinked
A4XOH.
Figure 4 shows the absorption spectrum of an A2X thin film
as a function of the immersion time in an acetone bath. For a
short contact time, the spectrum is essentially unchanged.
After 24 h of continuous immersion, the film was partially de-
Figure 5. AFM images of a film of A2X spin-coated at 900 rpm: a) as depos-
ited; b) after photocrosslinking; c) after acetone washing. Film spun at
2000 rpm: d) after acetone washing; e) highlight of delamination plaques
(the black lines are hand drawn and corresponds to the boundary of different
plaques) Figure S6 of the Supporting Information shows the original image.
Figure 4. UV/Vis absorption spectrum of a thin film of squaraine A2X and
Irgacure 651 (3 % wt.) on top of a microscope slide as a function of the
immersion time in an acetone bath: after irradiation (solid line), after 1 h
(dotted line) and 24 h (dashed line).
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aration steps: a) pre-UV irradiation (Figure 5, a); b) post-UV irra- ture. Optimized devices based on simple anilino squaraines
diation (Figure 5, b); c) post-UV irradiation and acetone devel- have efficiencies on the order of 1 %. The introduction of hy-
opment (Figure 5, c). AFM is inherently insensitive to chemical droxyl functionalities to improve planarity (as in the case of
transformations not involving clear changes in topology. As derivatives A2XOH and A4XOH) and the use of PC71BM instead
such, the images of the pristine material (Figure 5, a) and irradi- of PC61BM were found to improve efficiencies to 3 %.[57]
ated compound (Figure 5, b) films only differ by a small increase
Table 2. Summary of OPC cell characterization data for pristine and UV cross-
in surface roughness. Conversely, when the film was washed
linked A2X, A2XOH and A4XOH-based BHJ cells.
with acetone (which removes some unreacted material), the
Ratio D/A Photo-cross-
linking
Voc [V]
J
sc [mA/cm2] FF [%] η [%][a]
AFM image is very different from the previous ones showing
200 nm-deep trenches and a steep increase in roughness (ap-
prox. 100 nm). This phenomenon is common for acrylic deriva-
tives that only weakly adhere to substrate. Radical polymeriza-
tion involves a sizeable shrinking in going from the monomer
to the corresponding polymer. If the monomer film is too thick
and/or adhesion is poor, the elastic stress connected with the
shrinkage is relaxed through the formation of islands and par-
tial delamination.
A2X
A2X
A2XOH
A2XOH
A4XOH
A4XOH
1:1
1:1
1:1
1:1
1:1
1:1
no
yes
no
yes
no
0.25
0.46
0.35
0.45
0.14
0.23
0.43
3.52
0.63
0.70
0.07
0.44
32
27
27
31
26
28
0.03
0.42
0.06
0.10
0.002
0.03
yes
[a] Average value of data generated using 4 cells.
Likewise, the high polarity imparted by the presence of the
acrylic groups promotes too large a phase segregation/crystalli-
zation of the squaraine domains. Indeed, the powder X-ray
spectra shown in Figure 6 reveal a certain crystallinity in pristine
films. Interestingly, in the case of derivative A2X, the UV curing
process improves, rather than degrades, performance. More-
over, such a trend is common to all compounds we investi-
gated, although for both A2XOH and A4XOH, performance lev-
els remained poor. If we look again at Figure 6, the XRD pattern
shows that the photoinduced crosslinking leads to complete
amorphisation. This is consistent with previous observations
made by the McCulloch group when studying photocrosslinked
liquid crystalline materials for transistors. The same authors also
demonstrated that the photopolymerization process leads to
impaired charge mobility.[59]
A steep increase in the spinning speed (2000 rpm) enabled
the preparation of smoother films (Figure 5, d). In this case, the
acetone washing did not appear to remove unreacted material.
Nonetheless, the AFM image still shows the formation of pla-
ques (Figure 5, d), separated by trenches where the substrate is
directly exposed (such an effect is emphasised in Figure 5 (e)
where we manually highlighted plaque boundaries with a black
line). In short, in very thin films we do not observe the forma-
tion of island (with the connected high roughness) but we still
have delamination. Such behaviour was common to all the
squaraines we tested. The use of adhesion promoters and/or
extension to donor compounds other than squaraines could
mitigate the problem and is currently under investigation.
Photovoltaic Properties Preliminary Tests
We preliminarily tested the OPV performance of acrylic deriva-
tives as the donor component in a BHJ configuration having
PC61BM as the acceptor. Our cells were fabricated in an ambient
atmosphere by spin-coating squaraine:PC61BM 1:1 wt:wt ratio
solutions in CHCl3 onto ITO-coated glass substrates, which were
modified by evaporation of a MoOx layer as a hole extraction/
electron-blocking layer. After drying, the cells were then com-
pleted by sequential thermal vacuum deposition of LiF and Al
as the cathode. In all of the cases we compared the perform-
ance of identical devices, with and without UV exposure. The
device active area is 6 mm2 and the data discussed are the
result of data averaging from four different devices.
As with our previous work on developing different post-proc-
essing strategies,[29] we consistently worked with a 1:1 do-
nor:acceptor ratio (wt:wt) and used PC61BM instead of the bet-
ter performing PC71BM. Indeed, this paper focuses on the pho-
tocrosslinking process rather than on the achievement of high
efficiencies. The latter would require a systematic study of ac-
tive layer compositions and post-deposition annealing proc-
esses; such efforts are beyond the scope of this paper.
Figure 6. θ-2θ XRD scans of pristine and photocrosslinked A2XOH and
A4XOH squaraines on Si/SiO2.
As the same is likely true for our molecules as well, we specu-
late that, in the case of A2X, the sizeable improvement in OPV
performance may be related to a polymerization-induced segre-
gation of PCBM, which leads to improved morphology. Figure 7
shows the I–V characteristics along with the monochromatic
External Quantum Efficiency (EQE) of A2X:PC61BM cells after UV
Table 2 shows all the relevant device performance parame- exposure. The spectrum clearly shows the common tendency
ters (see also Figure S5 of the Supporting Information). The first of such squaraine derivatives to aggregate. In particular in this
relevant result is that, in all cases, performance is inferior to case, the band at 550–600 nm can be attributed to an H-aggre-
those generally reported for similar compounds in the litera- gate.[47]
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deposition treatment that, in principle, does not rely on the
device surface area. Not only were the materials still active after
the photocrosslinking process but, for the first time in the litera-
ture, we observed improved conversion efficiencies upon irradi-
ation. The maximum efficiencies we obtained (≈ 0.5 %) are
lower than the state of the art for squaraine derivatives (≈ 5.0 %
for optimized structurally complex derivatives and around 1 %
for simple anilino squaraines). We carried out a detailed mor-
phological analysis of the active layer before and after crosslink-
ing, showing that the main issue affecting overall device per-
formance is a delamination process stemming from shrinkage
of the squaraine layer upon polymerization. The use of im-
proved derivatives having larger conjugated cores should dra-
matically lessen the impact of such a phenomenon thereby
making more competitive efficiencies possible.
Experimental Section
General: All chemicals were purchased from Aldrich and used with-
out purification. Chromatographic purifications were performed us-
ing Merck 9385 silica gel, pore size 60 Å (230–400 mesh). 1H and 13
C
NMR spectra were recorded using a Bruker AMX-500 spectrometer
operating at 500 and 125.70 MHz, respectively. Coupling constants
are presented in Hz.
Figure 7. Top: I–V characteristics of a A2X:PC61BM device after exposure to
UV light. Bottom: EQE data for the device. The device structure is ITO/MoOx/
A2X:PC61BM/LiF/Al.
2-[Ethyl(phenyl)amino]ethyl Cinnamate (3): 2-[Ethyl(phenyl)-
amino]ethanol (1) (0.932 g, 5.64 mmol) is dissolved in toluene
(100 mL). Cinnamic acid (0.918 g, 6.20 mmol) and p-toluenesulfonic
acid (0.057 g, 0.56 mmol) are added to the solution and the mixture
is refluxed for 20 h in a Dean–Stark apparatus. Solvent is evaporated
at reduced pressure and the obtained viscous oil is redissolved in
dichloromethane, washed with brine and aqueous potassium carb-
onate. The organic phase is dried on magnesium sulfate, solvent is
evaporated at reduced pressure and the crude is purified through
a silica gel column (CH2Cl2/toluene, 8:2) to afford the pure material
as a viscous yellow oil (yield 57 %). 1H NMR (CDCl3): δ = 7.70 (d, J =
The shape of the I–V curve quite clearly indicates a serious
problem of shunt resistance, an issue connected with severe
contact resistance. We believe that such behaviour – which
more or less affects all the cells we made – is the direct conse-
quence of the delamination process highlighted in Figure 5 (e).
This behaviour seems to be a characteristic limitation of the
MoOx/squaraine interface. Thus far, we have not been able to 16.03 Hz, 1 H), 7.53 (m, 2 H), 7.41 (m, 3 H), 7.25 (t, J = 8.86 Hz, 2 H),
6.79 (d, J = 8.03 Hz, 2 H), 6.72 (t, J = 7.30 Hz, 1 H), 6.45 (d, J =
16.00 Hz, 1 H), 4.39 (t, J = 6.50 Hz, 2 H), 3.65 (t, J = 6.48 Hz, 2 H),
3.46 (quart., J = 7.06 Hz, 2 H) ppm. 13C NMR (CDCl3): δ = 167.78,
148.54, 146.05, 135.24, 131.26, 130.26, 129.79, 129.00, 118.67,
117.13, 112.94, 62.77, 49.80, 46.16, 13.19 ppm.
overcome this difficulty. The use of an adhesion promoter,
along with the extension of the approach to molecules having
a larger conjugated backbone, might be an efficient way to
improve device characteristics.
It should be noted that the highest efficiency we obtained
(0.42 %) is nearly one order of magnitude higher than that re-
ported (0.029 %)[36] for the only other example of photocross-
linked, small molecule-based OPV devices we could find. We
would, however, stress that the previous report remains an im-
portant achievement as the whole process was directly carried
out on roll-to-roll equipment.[36]
Squaraine (C2X): Compound 3 (0.730 g, 2.47 mmol) is dissolved in
an n-butanol/toluene, 1:1 mixture (30 mL). Squaric acid (0.140 g,
1.23 mmol) is added and the mixture is refluxed for 5 h under a
Dean–Stark trap to remove the water formed. Solvent is removed
at reduced pressure and the solid is triturated with diethyl ether,
1
affording the product as a black solid (27 %). H NMR (CDCl3): δ =
8.43 (d, J = 9.13 Hz, 4 H), 7.68 (d, J = 16.00 Hz, 2 H), 7.51 (m, 4 H),
7.38 (m, 6 H), 6.87 (d, J = 9.22 Hz, 4 H), 6.39 (d, J = 16.06 Hz, 2 H),
4.44 (t, J = 6.03 Hz, 4 H), 3.82 (t, J = 6.08 Hz, 4 H), 3.62 (quart., J =
6.94 Hz, 4 H), 1.30 (t, J = 7.12 Hz, 6 H) ppm. 13C NMR (126 MHz,
CDCl3): δ = 190.65, 184.02, 167.57, 154.50, 146.85, 134.92, 134.42,
131.48, 129.82, 129.10, 121.14, 117.86, 113.39, 62.14, 49.84, 46.88,
13.28 ppm. C42H40N2O6 (668.79): calcd. C 75.43, H 6.03, N 4.19;
found C 75.39, H 6.13, N 4.03.
Conclusions
In conclusion, we have designed, synthesized and characterized
two cinnamic esters and four acrylic esters functionalized squar-
aines. Cinnamic derivatives did not display any photoinduced
crosslinking behaviour. Conversely, all of the acrylic derivatives
behaved as negative photoresists strongly improving the sol-
vent resistance of thin films. We preliminarily tested such mate-
rials in BHJ solar cells along with PCBM as the acceptor. In this
(2E,2′E)-2,2′-(Phenylazanediyl)bis(ethane-2,1-diyl)bis(3-phenyl-
acrylate) (4): 2,2′-(Phenylazanediyl)diethanol (2) (30.000 g,
165.50 mmol) is dissolved in toluene (300 mL). Cinnamic acid
(73.000 g, 364.20 mmol) and p-toluenesulfonic acid (5.600 g,
application our crosslinkable materials make possible a post- 16.50 mmol) are added to the solution and the mixture is refluxed
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for 200 h in a Dean–Stark apparatus. Solvent is evaporated at re-
duced pressure and the obtained viscous oil is dissolved in dichloro-
methane, washed with brine and aqueous potassium carbonate.
The organic phase is dried on magnesium sulfate, solvent is evapo-
rated at reduced pressure and the crude is purified through a silica
gel column (CH2Cl2/toluene, 8:2) to afford the pure material as a
viscous yellow oil (yield 48 %). 1H NMR (CDCl3): δ = 7.69 (d, J =
16.02 Hz, 2 H), 7.52 (m, 4 H), 7.39 (m, 6 H), 7.28 (m, 3 H), 6.87 (d,
J = 8.17 Hz, 2 H), 6.77 (t, J = 7.25 Hz, 1 H), 6.45 (d, J = 15.98 Hz, 2
H), 4.43 (t, J = 6.26 Hz, 4 H), 3.77 (t, J = 6.30 Hz, 4 H) ppm. 13C NMR
(CDCl3): δ = 167.32, 147.75, 145.74, 134.75, 130.85, 129.98, 129.35,
128.62, 118.12, 117.58, 112.73, 62.14, 50.40 ppm.
1H NMR (CDCl3): δ = 8.42 (d, J = 9.12 Hz, 4 H), 6.93 (d, J = 9.23 Hz,
4 H), 6.07 (s, 4 H), 5.59 (s, 4 H) 4.41 (t, J = 5.82 Hz, 8 H), 3.86 (m
broad, 8 H), 1.91 (s, 12 H) ppm. 13C NMR (CDCl3): δ = 192.58, 183.75,
167.96, 154.86, 136.45, 134.51, 127.48, 121.74, 113.64, 62.12, 50.32,
19.16 ppm. C40H44N2O10 (712.80): calcd. C 69.28, H 6.52, N 3.94;
found C 69.39, H 6.58, N 4.06.
6-Bromohexyl Acrylate: In a three-necked round bottomed flask
equipped with a calcium chloride trap triethylamine (3.04 g,
30.05 mmol) and 6-bromohexanol (5.00 g, 27.61 mmol) are dis-
solved in dry toluene (30 mL). The mixture is then cooled to –10 °C
via salt and ice bath, and a solution of acryloyl chloride (2.72 g,
30.05 mmol) in dry toluene (10 mL) is added dropwise. The reaction
warmed to room temperature and stirred for 48 h. The organic
phase is subsequently washed with a 5 % w/w aqueous solution of
sodium carbonate, a 5 % w/w aqueous hydrochloric acid solution
and with brine. The organic phase is then dried on magnesium
sulfate and the solvent is evaporated at reduced pressure to afford
the product as a yellowish oil (yield 77 %). 1H NMR (CDCl3): δ = 6.40
(d, J = 17.37 Hz, 1 H), 6.12 (dd, J = 17.35, 10.43 Hz, 1 H), 5.82 (d, J =
10.43 Hz, 1 H), 4.16 (t, J = 6.63 Hz, 2 H), 3.41 (t, J = 6.82 Hz, 2 H),
1.88 (quint, J = 6.80 Hz, 2 H), 1.69 (quint, J = 6.73 Hz, 2 H), 1.45
(quint, J = 6.72 Hz, 2 H), 1.41 (quint, J = 6.77 Hz, 2 H) ppm. 13C NMR
(CDCl3): δ = 166.28, 130.55, 128.54, 64.39, 33.68, 32.60, 28.45, 25.29,
25.17 ppm.
Squaraine (C4X): Compound 4 (5.000 g, 11.32 mmol) is dissolved
in 2-propanol (40 mL). Squaric acid (0.645 g, 5.66 mmol) and triethyl
orthoformate (10 mL) are added and the mixture is refluxed for 4 h.
The mixture is then cooled in an ice bath and a shiny precipitate
is formed. The precipitate is filtered under reduced pressure, and
crystallized from ethanol to give the title compound as a blue pow-
der (yield 10 %). 1H NMR (CDCl3): δ = 8.48 (d, J = 9.12 Hz, 4 H), 7.67
(d, J = 16.02 Hz, 4 H), 7.50 (m, 4 H), 7.37 (m, 6 H), 6.91 (d, J =
9.20 Hz, 4 H), 6.40 (d, J = 15.99 Hz, 4 H), 4.49 (t, J = 5.89 Hz, 4 H),
3.93 (t, J = 5.92 Hz, 4 H) ppm. 13C NMR (CDCl3): δ = 191.00, 183.51,
167.56, 155.00, 146.96, 134.87, 134.75, 131.50, 129.82, 129.13,
121.53, 117.75, 113.88, 61.96, 50.82 ppm. C60H52N2O10 (961.08):
calcd. C 74.98, H 5.45, N 2.91; found C 74.71, H 5.68, N 2.82.
6,6′-(3-Hydroxyphenylazanediyl)-bis(hexane-6,1-diyl) Diacryl-
ate (10): 3-Aminophenol (compound 7, 409 mg, 3.75 mmol), 6-
bromohexyl acrylate (1.84 g, 7.82 mmol) and finely ground sodium
carbonate (1.06 g, 10.0 mmol) are added to 2-propanol (10 mL). The
mixture is refluxed for 20 h. The precipitate is filtered away and the
organic phase is evaporated at reduced pressure to afford a brown
oil, which is taken filtered through a plug of silica with a heptane/
ethyl acetate, 2:1 mixture. After solvent evaporation the product is
obtained as a light yellow oil (650 mg, 42 %) yield. The compound
very easily spontaneously polymerized and was directly used in the
next step without further characterization.
Squaraine (A2X): 2-[Ethyl(phenyl)amino]ethanol (1) (3.870 g,
23.40 mmol) is dissolved in toluene (80 mL). Triethylamine (2.610 g,
25.80 mmol) is added to the solution and the mixture cooled down
to 0 °C via ice bath. A solution of methacryloyl chloride (2.680 g,
25.60 mmol) in toluene (20 mL) is added dropwise to the mixture.
Reaction is stirred for 12 h at room temperature. The white precipi-
tate is filtered away and the organic phase is concentrated at re-
duced pressure. The crude mixture is filtered through a silica plug
using CH2Cl2 as a solvent, affording pale yellow oil as intermediate.
This intermediate is immediately dissolved in 2-propanol (10 mL),
to which both squaric acid (0.570 g, 5.00 mmol) and triethyl ortho-
formate (1 mL) are added. The mixture is refluxed for 3 h and then
cooled in an ice bath and a shiny precipitate is formed. The precipi-
tate is isolated by suction filtration and crystallized from methanol
Squaraine (A4XOH): Crude compound 10 (0.620 g, 1.5 mmol) is
dissolved in 2-propanol (10 mL). Squaric acid (0.86 g, 0.75 mmol)
and triethyl orthoformate (1.5 mL) are added and the mixture is
refluxed for 4 h. Volatiles are evaporated at reduced pressure and
the resulting dark wax is taken up with 2-propanol to give a bright
shiny precipitate that is isolated by filtration and crystallized from
2-propanol (101 mg, yield 15 %). The squaraine was isolated as a
10:3 trans/cis mixture of isomers with respect to the position of the
1
to afford the title compound as a blue powder (yield 7 %). H NMR
(CDCl3): δ = 8.41 (d, J = 9.15 Hz, 4 H), 6.84 (d, J = 9.22 Hz, 4 H), 6.08
(s, 2 H), 5.59 (s, 2 H), 4.39 (t, J = 5.93 Hz, 4 H), 3.79 (t, J = 5.98 Hz,
4 H), 3.59 (quart., J = 7.10 Hz, 4 H), 1.92 (s, 6 H), 1.28 (s, 6 H) ppm.
13C NMR (CDCl3): δ = 190.73, 184.03, 168.03, 154.48, 136.53, 134.37,
127.40, 121.13, 113.34, 62.41, 49.66, 46.69, 19.17, 13.19 ppm.
C32H36N2O6 (544.65): calcd. C 70.57, H 6.66, N 5.14; found C 70.67,
H 6.59, N 5.20.
1
hydroxyl residue. H NMR (500 MHz, CDCl3): δ = 11.44 (s, 2 H), 7.92
(d, J = 9.21 Hz, 2 H), 6.49 (dd, J = 17.34, 1.34 Hz, 2 H), 6.34 (dd, J =
9.22, 1.62 Hz, 2 H), 6.16–6.08 (m, 4 H), 5.88 (dd, J = 10.43, 1.17 Hz,
2 H), 4.16 (t, J = 6.58 Hz, 8 H), 3.42 (t, J = 7.30 Hz, 8 H), 1.78–1.16
(m, 12 H), 1.62 (s broad, 8 H), 1.52–1.38 (m, 12 H) ppm. 13C NMR
not recorded for lack of solubility. C52H89N2O10 (902.28): calcd. C
70.88, H 7.78, N 3.18; found C 71.01, H 7.65, N 3.15.
Squaraine (A4X): 2,2′-(Phenylazanediyl)diethanol (2) (2.200 g,
12.29 mmol) is dissolved in toluene (80 mL). Triethylamine (2.720 g,
26.60 mmol) is added to the solution and the mixture cooled down
to 0 °C via ice bath. A solution of methacryloyl chloride (2.680 g,
25.60 mmol) in toluene (20 mL) is added dropwise to the mixture.
Reaction is brought out for 12 h at room temperature. The white
precipitate is filtered away and the organic phase reduced by evap-
orating the solvent at reduced pressure. The crude is filtered
through a silica plug using CH2Cl2 as a solvent, affording a pale
3-(Octylamino)phenol (8): 3-Aminophenol (35.00 g, 320.00 mmol),
bromooctane (62.31 g, 323.10 mmol) and finely ground sodium
carbonate (38.42 g, 350.12 mmol) are added to 2-propanol (70 mL).
The mixture is refluxed for 10 h. The precipitate is filtered away and
the organic phase is evaporated at reduced pressure to afford
yellow oil as intermediate. This transient species is immediately dis- brown oil, which is then triturated in a heptane/ethyl acetate, 4:1
solved in 2-propanol (10 mL), to which both squaric acid (0.500 g,
4.40 mmol) and triethyl orthoformate (1 mL) are added. The mixture
is refluxed for 3 h and then cooled in an ice bath. The dark precipi-
tate is filtered away on a Hirsch funnel and crystallized from ethanol
to afford the title compound as a dark green powder (yield 27 %).
mixture. The solid is filtered away and the crude is purified by col-
umn chromatography on silica (eluent: heptane/ethyl acetate, 4:1).
After solvent evaporation the product is obtained as a white pow-
der (yield 37 %). 1H NMR (CDCl3): δ = 7.83 (s broad, 1 H), 7.20 (t, J =
8.02 Hz, 1 H), 6.21 (d, J = 8.15 Hz, 1 H), 6.18 (d, J = 8.08 Hz, 1 H),
Eur. J. Org. Chem. 0000, 0–0
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7
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
6.09 (t, J = 2.26 Hz, 1 H), 3.21 (t, J = 7.65 Hz, 2 H), 1.55 (quint, J = methanol, and acetone sequentially, and finally in a UV-ozone
7.55 Hz, 2 H), 1.35.1.25 (m, 10 H), 0.89 (t, J = 7.08 Hz, 3 H) ppm.
cleaner for 30 min under ambient atmosphere. MoOx was depos-
ited as described, followed by the active layer. All blend solutions
of squaraine + PCBM were stirred for 1 h at 40 °C and then soni-
cated for 1 at 40 °C in the dark. The active layer was then obtained
by spin-coating the blends at 2000 rpm for 30 sec in ambient at-
mosphere. Where necessary, sample areas for contacts were
cleaned with chloroform and a cotton swab, and then transferred
to the glove-box in nitrogen atmosphere for the electrode evapora-
tion or for UV-treatment. Samples that undergo UV treatment were
exposed for 30 min to the direct unfiltered light of a portable TLC
lamp working at the 365 nm wavelength and kept at a 2 cm dis-
tance from the film. Cathode deposition was carried out according
to the same procedure used for UV-treated and pristine samples.
The cathode growth rates used were 0.1 Å/s for LiF (Acros, 99.98 %)
and approx. 1.5 Å/s for Al (Sigma–Aldrich, 99.999 %), with a cham-
ber pressure of 1.1 × 10–6 Torr. The cathodes were deposited
through a shadow mask with two 2 mm strips perpendicular to the
two patterned ITO (about 3 mm) strips to make four devices per
substrate. Cells were not encapsulated. Atomic force microscopic
(AFM) images were obtained using a JEOL-5200 Scanning Probe
Microscope with silicon cantilevers in the tapping mode, using
WinSPM Software.
6-[(3-Hydroxyphenyl)(octyl)amino]hexyl Acrylate (9): Com-
pound 8 (0.94 g, 4.25 mmol), bromohexyl acrylate (1.30 g,
5.52 mmol) and finely ground sodium carbonate (0.90 g, 8.50 mmol)
in 2-propanol (5 mL) is refluxed for 74 h. The precipitate is filtered
away and the organic phase is evaporated at reduced pressure to
afford a brown oil which is purified by column chromatography
(eluent: petroleum ether/ethyl acetate, 5:1). After evaporation the
product is obtained as a yellowish oil (yield 40 %). The compound
very easily and spontaneously polymerized and was directly used
in the next step without further characterization.
Squaraine (A2XOH): The crude compound 9 (0.600 g, 1.6 mmol) is
dissolved in 2-propanol (10 mL). Squaric acid (0.90 g, 0.8 mmol) and
triethyl orthoformate (1.5 mL) are added and the mixture is refluxed
for 4 h. The volatiles are evaporated at reduced pressure and the
resulting dark wax is taken up with 2-propanol to give a bright
shiny precipitate that is isolated by filtration and crystallized from
2-propanol (86 mg, yield 15 %). The squaraine was isolated as a
10:3 trans cis mixture of isomers with respect to the position of the
hydroxyl residue. 1H NMR (CDCl3): δ = 12.09 (s, 2 H), 7.87 (d, J =
9.11 Hz, 2 H), 6.41 (dd, J = 17.32, 1.18 Hz, 2 H), 6.30 (dd, J = 9.06,
1.62 Hz, 2 H), 6.16–6.08 (m, 4 H), 5.83 (dd, J = 10.42, 1.17 Hz, 2 H),
4.16 (t, J = 6.56 Hz, 4 H), 1.73–1.59 (m, 12 H), 1.48–1.38 (m, 12 H),
1.35–1.22 (m, 24 H), 0.89 (t, J = 6.59 Hz, 6 H) ppm. 13C NMR (CDCl3):
δ = 182.9, 172.2, 166.3, 164.4, 156.3, 132.4, 130.7, 128.5, 109.8, 107.5,
98.5, 64.3, 51.6, 51.4, 31.7, 29.3, 28.5, 27.7, 27.6, 27.0, 26.6, 25.8, 22.6,
14.1 ppm. C50H72N2O8 (829.13): calcd. C 75.34, H 9.10, N 3.51; found
C 74.96, H 9.48, N 3.11.
BHJ OPV I–V Characterization: Device evaluation was performed
at 298 K using a Class A Spectra-Nova Technologies solar cell analy-
ser having a xenon lamp that simulates AM1.5G light from 400–
1100 nm. The instrument was calibrated with a monocrystalline Si
diode fitted with a KG3 filter to bring spectral mismatch to unity.
The National Renewable Energy Laboratory (NREL) calibrated the
calibration standard. Data are corrected for spectral mismatch. Four-
point contacts were made to the substrate with copper alligator
clips. A mask isolated individual devices during testing to avoid
current collection from adjacent devices and edge effects.
Electrochemical Characterization: Squaraines were dissolved
(concentration about 10–4
M) in the supporting electrolyte that was
a 0.1
M solution of tetrabutylammonium hexafluorophosphate
(Fluka, electrochemical grade, ≥ 99.0 %) in anhydrous acetonitrile
(Aldrich, 99.8 %). Cyclic Voltammetry (CV) was carried out at scan
rate of 50 mV/s, using a PARSTA2273 potentiostat in a single cham-
ber three electrodes electrochemical cell in a glove box filled with
Argon ([O2] ≤ 1 ppm). The working, counter, and the pseudo-refer-
ence electrodes were an Au pin, a Pt flag and a Ag/AgCl wire, re-
spectively. The Ag/AgCl pseudo-reference electrode was externally
Supporting Information (see footnote on the first page of this
article): Additional UV/Vis spectra, TGA traces, cyclic voltammetry
characterization, OPV cells characterization and copy of the NMR
spectra is given.
Acknowledgments
calibrated by adding ferrocene (1 mM) to the electrolyte. Values
Financial support from Fondazione Cariplo, Milano, Italy
through grants 2010-0564 and 2011-1832 and from the Euro-
pean Community through grant 607232
reported in Table 1 were obtained using the following equation:
EHOMO [eV] = –(Eox – EFc/Fc+) – 4.80 eV, where Eox and EFc/Fc+ are the
half wave oxidation potentials for the squaraine sample and the
ferrocene against the Ag/AgCl reference electrode, while the value
– 4.8 eV is the HOMO energy of ferrocene against vacuum. The
LUMO energy levels were calculated with the HOMO levels obtained
by CV measurements and the optical band gap (Egopt) obtained by
(Thinface) are gratefully acknowledged.
Keywords: Photochemistry · Photopolymerization · Solar
cells · Electrochemistry · Donor–acceptor systems ·
Squaraines
UV/Vis measurements: ELUMO [eV]=Egopt + EHOMO
.
BHJ OPV Device Fabrication and Thin Film Characterization: De-
vices were fabricated by spin-coating blends of squaraine + PCBM,
sandwiched between a transparent anode and an Al cathode. The
anode consisted of glass substrates pre-coated with indium tin ox-
ide (ITO), modified by thermal evaporation of MoOx. The evapora-
tion rate was 0.6 A/s up to a total thickness of 6 nm. Evaporation
was followed by 5 min of UV irradiation to realign frontier orbitals.
The cathode consisted of lithium fluoride (about 0.6 nm) capped
with aluminium (about 120 nm). Details of these procedures and
the equipment have been described previously.[60] Before device
fabrication, the commercial ITO film on glass substrates (1.1 mm)
was patterned with aqueous HCl solution. The ITO-coated (150 nm
thick films) glass substrates were then cleaned by ultrasonic treat-
ment in aqueous detergent, deionized water, isopropyl alcohol,
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Received: May 3, 2016
Published Online: ■
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Organic Photovoltaics
The use of functional photoresists
based on acrylic functionalized squara-
ines enables the preparation of solvent
resistant small molecule-based Or-
ganic Bulk Heterojunction solar cells.
The photocrosslinking process im-
proves conversion efficiencies with re-
spect to the corresponding unexposed
devices.
R. Turrisi, L. Mascheroni, M. Sassi,
M. Rooney, N. Buccheri, R. Ruffo,
A. Facchetti, L. Beverina* .............. 1–10
Synthesis and Characterization of
Squaraine-Based
Photocrosslink-
able Resists for Bulk Heterojunction
Solar Cells
DOI: 10.1002/ejoc.201600552
Eur. J. Org. Chem. 0000, 0–0
www.eurjoc.org
10
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim